Direct Electroless Palladium Plating on Copper

ABSTRACT

A method of providing a direct electroless palladium deposit on a copper surface is described. The method comprises the steps of (a) catalyzing the copper surface by applying a pre-dip composition to the copper surface, the pre-dip composition comprising a reducing agent; and thereafter (b) contacting the catalyzed copper surface with an electroless palladium composition to deposit a layer of palladium on the copper surface.

FIELD OF THE INVENTION

The present invention relates generally to direct electroless palladium plating on copper substrates.

BACKGROUND OF THE INVENTION

Electroless deposition is a process for depositing a thin layer or layers of a material(s) onto a substrate. Electroless deposition is typically accomplished by immersing the substrate in a bath that contains ions of the material to be deposited along with a chemical reducing agent, whereby some of the ions precipitate onto the substrate surface. In contrast to electroplating processes, electroless deposition does not normally require an externally applied electrical field to facilitate deposition. Thus, one advantage of electroless plating processes is that it can be selective, i.e., the material can be deposited only onto areas that demonstrate appropriate electrochemical properties, and thus, local deposition can be performed on areas that have been pretreated or catalyzed.

A molded interconnect device (MID) is an injection-molded thermoplastic element with integrated electronic circuit traces. MIDs provide new opportunities for miniaturization of complex devices and integration of functionalities into one component reduces assembly costs and product volume. MIDs combine a high temperature plastic substrate (or housing) with circuitry into a single element through selective metallization. In both printed circuit board (PCB) and MID manufacturing processes, electronic components are bonded to selected bonding areas of a copper structure produced on one or both sides of a substrate. Such interconnection must be reliable in terms of bond strength.

Wire bonding is one of the preferred processes for connecting the chip in interconnect packages. One of the main wire bonding processes used for IC-substrates is gold wire bonding, in which a gold wire is bonded on a layer of electrolytically deposited nickel and gold. Alternatively, gold wire may be bonded onto a surface of nickel, palladium and gold.

The mechanical reliability of wire bonds in microelectronic packages depends on the formation and development of intermetallic compounds at the interface between the bond wedge and the bond pad on the substrate, which is needed for successful bonding. Bonding either gold or copper-wires onto a copper bond pad surface is difficult mainly because of the tendency of the copper metallization to oxidize.

Wire bonding portions are typically made of copper. If they remain bare or are externally exposed to the atmosphere and humidity, soldering and wire bonding properties of the copper layers can deteriorate due to oxidation or corrosion of the surface. In order to maintain soldering and/or wire bonding properties, bare or exposed copper layers are typically electroplated or electrolessly plated with nickel. The plated nickel layer protects the copper from a corrosive environment for an extended period of time. In addition, the nickel layer protects the copper from being dissolved by solder during the soldering assembly step by functioning as a diffusion barrier layer. In addition, the plated nickel layer also acts as an interfacial film for prevention the copper layer and a subsequently plated gold layer, from diffusing into each other.

For many years, electroless nickel/immersion gold (ENIG) has been the default finish of choice for many electronics applications. However, while ENIG has enjoyed widely use as a surface finish, the use of nickel can cause a widespread allergic contact dermatitis among users. Thus, a nickel-free surface finish is now required by many original equipment manufacturers (OEMs).

U.S. Pat. No. 6,794,288 to Kolics et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method for electroless formation of phosphorus-containing metal films, such as cobalt-tungsten-phosphorus (Co—W—P) systems on copper substrates with palladium free activation.

The inventors of the present invention have found that palladium final finishes provide ideal surfaces for soldering, wire bonding and molded interconnect devices (MID) applications with the advantage of completely replacing nickel/gold finishes. However, direct electroless palladium plating on copper deposits can be hard to initiate. This has become a major hurdle and has slowed down the process to replace nickel.

Thus it would be desirable to provide an improved electroless palladium plating process that allows for direct palladium plating on copper substrates.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for direct electroless palladium plating on copper substrates.

It is another object of the present invention to provide a palladium final finish on a copper or copper alloy substrate for soldering, wire bonding and molded interconnect devices (MID) applications.

It is still another object of the present invention to provide an electroless palladium deposit to replace a nickel/gold finish.

To that end, in one embodiment, the present invention relates generally to a method of providing an electroless palladium deposit on a copper surface, the method comprising the steps of:

a) catalyzing the copper surface by applying a pre-dip composition to the copper surface, the pre-dip composition comprising a reducing agent; and thereafter

b) contacting the catalyzed copper surface with an electroless palladium composition to deposit a layer of palladium on the copper surface.

The electroless palladium, deposit can then be soldered to or wire bonded to.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors of the present invention have discovered an improved process for catalyzing the copper surface through the use of a pre-dip solution, which as makes direct electroless palladium plating on copper possible.

In one embodiment, the present invention relates generally to a method of providing an electroless palladium deposit on a copper surface, the method comprising the steps of:

a) catalyzing the copper surface by applying a pre-dip composition to the copper surface, the pre-dip composition comprising a reducing agent; and thereafter

b) contacting the catalyzed copper surface with an electroless palladium composition to deposit a layer of palladium on the copper surface.

The copper surface may comprise copper or copper alloy and may be a copper or copper alloy surface used in a printed circuit board or molded interconnect device.

As described herein, the use of the electroless palladium produces a product that does not contain nickel or nickel alloy and in which the electroless palladium is deposited directly on the underlying copper substrate. Thus, the use of the electroless palladium as described herein replaces the more commonly used nickel/gold finishes of the prior art.

The present invention utilizes a pre-dip composition comprising a reducing agent that catalyzes the copper surface for the subsequent deposition of electroless palladium thereon. In a preferred embodiment, the reducing agent comprises a boron reducing agent selected from the group consisting of alkyl boranes, amine boranes, borane complexes, boron hydride compounds and combinations of one or more of the foregoing. In addition, it is also believed that phosphorus-based reducing agents, including hypophosphorous-acid-based reducing agents and their salts would be usable in the pre-dip composition.

Examples of suitable alkyl boranes include, but are not limited to, trimethylborane, methoxydiethylborane, and dibutylboron triflate. Examples of suitable amine boranes include but are not limited to dimethylamine borane, t-butylamine borane, pyridine borane, triethylamine borane, tri-ethylamineborane-1,3-diaminopropane complex, ethylenediamine borane, 5-ethyl-2-methylpyridine borane. Of these amine boranes, dimethyamine borane is preferred. Examples of suitable borane complexes include, but are not limited to, borane-tetrahydrofuran complex, dimethyl sulfide borane, and N,N-diethylaniline borane, morpholine borane, and piperazine borane, among others. In one preferred embodiment, the reducing agent comprises dimethyamine borane.

The reducing agent is preferably present in the pre-dip composition at a concentration of between about 2 and about 20 g/L, more preferably between about 5 and 15 g/L and most preferably at a concentration of between about 8 and about 10 g/L. In addition, the bath of the pre-dip composition is preferably maintained at a temperature of between about 20 and about 30° C., more preferably at about room temperature. The copper surface is contacted with the pre-dip composition in the bath for a period of time to catalyze the copper surface which is typically between about 30 seconds and about 2 minutes. The copper surface may be contacted with the pre-dip composition by spray coating, curtain coating or immersion. In a preferred embodiment, the copper surface is contacted with the pre-dip composition by immersing the copper surface in the pre-dip composition for the desired period of time.

Once the copper surface has been contacted with the pre-dip composition to catalyze the surface, the now catalyzed copper surface is contacted with an electroless palladium plating solution.

The electroless palladium plating solution preferably comprises:

a) a source of palladium ions;

b) one or more complexing agents; and

c) a reducing agent.

The source of palladium ions is preferably a palladium salt which typically comprises a salt such as palladium chloride, palladium sulfate, palladium nitrate, palladium nitrite, and palladium acetate, by way of example and not limitation. The concentration of the palladium salt in the pre-dip is preferably in the range of about 2 to about 6 g/L, more preferably about 3 to about 5 g/L.

The one or more complexing agents may typically include one or more nitrogenated complexing agents and suitable nitrogenated complexing agents include primary, secondary and tertiary amines as well as polyamines. For example, nitrogenated complexing agents may include, but are not limited to, ethylenediamine, 1,3-diaminopropane, 1,2-bis(3-amino-propyl-amino)-ethane, 2-diethyl-amino-ethylamine, and diethylene triamine. Other nitrogenated complexing agents include diethylene-triamine-penta-acetic acid, nitro-acetic acid, N-(2-hydroxyethyl)-ethylenediamine, ethylenediamine-N,N-diacetic acid, 2-(dimethylamino)-ethylamine, 1,2-diamino-propylamine, 1,3-diamino-propylamine, 3-(methylamino)-propylamine, 3-(dimethylamino)-propylamine, 3-(diethylamino)-propylamine, bis-(3-aminopropyl)-amine, 1,2-bis-(3-aminopropyl)-alkylamine, diethylenetriamine, triethylenetetramine, tetraethylene-pentamine, penta-ethylene-hexamine, and combinations of one or more of the foregoing. In one embodiment, the nitrogenated complexing agent comprises ethylenediamine. The concentration of the one or more nitrogenated complexing agents in the electroless palladium solution is preferably in the range of between about 2 to about 8 g/L, more preferably between about 3 to about 6 g/L.

In addition, the one or more complexing agents may also comprise a compound containing a carboxylic acid group. The compound containing a carboxylic acid group is preferred because it forms a complex with the palladium ions. Examples of the compound containing a carboxylic acid group include, but are not limited to, citric acid, acetic acid, propionic acid, lactic acid, ortho-hydroxybenzoic acid, oxalic acid, malonic acid, succinic acid, maleic acid, tartaric acid, ortho-phthalic acid, diglycolic acid, thioglycolic acid, thiodiglycolic acid, glycine, methylglycine, dimethylglycine, anthranilic acid, picolinic acid, quinolinic acid and combinations of one or more of the foregoing. The concentration of the one or more complexing agents comprising a carboxylic acid group is preferably in the range of between about 8 and about 25 g/L, more preferably in the range of between about 10 and about 20 g/L.

The electroless palladium plating solution also preferably comprises a reducing agent and the reducing agent may preferably be a hypophosphite reducing agent. Other reducing agents such as formaldehyde, hydrazine and boron reducing agents may also be usable in the compositions described herein, depending on the chemistry of the plating bath.

The electroless palladium plating solution also preferably comprises a pH buffering agent, which may comprise a suitable acid such as formic acid, acetic acid, malonic acid, succinic acid or citric acid.

In addition, various solvents, including aliphatic alcohols as well as diols and polyols such as ethylene glycol and glycerine may also optionally, but preferably, be included in the electroless palladium plating solution. Mixtures of such solvents as well as blends with other solvents can also be used. The concentration of the solvent may be in the range of about 30 to about 50 g/Lm more preferably, about 35 to about 45 g/L.

The electroless palladium plating solution may also comprise other additives that are usable in electroless plating solutions, including, but not limited to brighteners, stabilizers, surfactants, by way of example and not limitation.

An example of a suitable electroless palladium plating solution that is usable in the process described herein is set forth in Table 1.

TABLE 1 Typical electroless palladium solution Component Concentration (g/L) Ethylenediamine 4 Palladium chloride 3 Sodium hypophosphite 5 Propionic acid 7 Glycine 1.0 Brightener 0.10

The pH of the electroless palladium solution is preferably above 4, more preferably in the range of about 4 to about 10.

The electroless palladium bath is typically maintained a temperature of between about 45 and about 60° C., more preferably at a temperature of between about 50 and about 55° C. The copper surface is contacted with the electroless palladium composition for a period of time to deposit the desired thickness of palladium. In one embodiment, the copper surface is contacted with the electroless palladium composition for about 2 to about 20 minutes, more preferably for about 5 to about 15 minutes. For example, using the plating bath described above in Table, immersion of a copper substrate in the electroless palladium composition for 4 minutes yielded an electroless palladium layer on the copper substrate having a thickness of about 8 microinches. The copper substrate is preferably contacted with the electroless plating composition by immersing the copper substrate in the electroless plating composition for the desired period of time at the desired temperature.

Example 1

A copper or copper alloy surface is first prepared for plating. For example, the copper or copper alloy surface may be prepared by treatment with an acid cleaner and then the surface copper oxides may be removed in a microetch bath.

Thereafter, the copper or copper alloy surface is contacted with a pre-dip composition comprising about 8 g/L of DMAB and then with an electroless palladium bath as described above in Table 1.

The copper or copper alloy surface is subjected to the following steps set forth in Table 2 to deposit an electroless palladium layer.

TABLE 2 Steps in direct electroless palladium plating in accordance with the present invention. Step Temperature (° C.) Time (minutes) Pd predip 25 1 Palladium plating bath 52 15 DI rinse RT 2 Dry n/a n/a Total Time (minutes) 18

Comparative Example 1

As in Example 1, the copper or copper alloy surface is prepared first prepared for plating. For example, the copper or copper alloy surface may be prepared by treatment with an acid cleaner and then the surface copper oxides may be removed in a microetch bath.

Thereafter, the copper or copper alloy surface is subjected to the following process steps as set forth in Table 3 to apply an ENIG layer on the copper surface.

TABLE 3 Typical steps in an ENIG process. Step Temperature (° C.) Time (minutes) Acid Pre-dip RT 1 Rinse RT 1 Ni initiator 43 3 Rinse RT 1 Acid Post Dip RT 1 Nickel plating bath 88 15 Rinse RT 2 Gold plating bath 85 10 Rinse RT 2 Dry n/a n/a Total time (minutes) 40

Thus, as compared with the ENIG process set forth in Comparative Example 1, the electroless palladium process described in Example 1 has fewer process steps, resulting in shorter production times. In addition, lower bath temperatures also translate into lower energy costs.

Furthermore, the use of cyanide in immersion gold baths has plagued the industry for many years, especially because treatment of waste water for cyanide is an expensive process. In addition, costly government licenses are required where cyanide is used in production. In contrast, the MID palladium process described herein is entirely cyanide free.

Solderability Testing:

Parts coated with electroless palladium and parts coated with ENIG were dipped into a tin/lead solder pot maintained at a temperature of 290° C. to check for solder wetting of the surface. Parts coated with electroless palladium and parts coated with ENIG both wet equally well.

Salt Spray Testing:

Parts coated with electroless palladium and parts coated with ENG were subjected to a 48-hour neutral salt spray test in accordance with ASTM 117-09 to demonstrate resistance to a hazardous environmental over a sustained period of time. The electroless palladium coated copper demonstrated greater visual resistance to the neutral salt spray test than the ENIG coated copper.

Sulfur Tarnish Testing:

Parts coated with electroless palladium and parts coated with ENIG were placed into a sealed 50° C. chamber in a sulfur chamber for 10 minutes. It was found that the electroless palladium coated parts offered the same tarnish protection as the ENIG coated parts.

Steam Aging:

Parts coated with electroless palladium, parts coated with ENIG and parts coated with silver were aged in a steam bath for 8 hours at 100° C. to evaluate the ability of the respective coatings to adhere to the copper surface after an extended period of immersion in the steam bath. All of the coated parts passed the steam aging test, with no delamination of the metallization from the substrate. In addition the electroless palladium coated part passed the same as the ENIG and the silver coated parts.

Thus it can be seen that the palladium process offers the same advantages as an ENIG final finish. The palladium finish gives desirable results for solderability, salt spray, sulfur tarnish, contact resistance, steam aging, paint and RF testing. In addition, palladium baths are cyanide free, unlike many of the widely used gold baths. With the electroless palladium process described herein, the process is simple and costs are significantly reduced due to the removal of expensive gold metal and the running cost of electroless nickel. Finally, production times are reduced by nearly 70% with use of palladium as compared with ENIG. 

What is claimed is:
 1. A method of providing an electroless palladium deposit on a copper surface, the method comprising the steps of: a) catalyzing the copper surface by applying a pre-dip composition to the copper surface, the pre-dip composition comprising a reducing agent; and thereafter b) contacting the catalyzed copper surface with an electroless palladium composition to deposit a layer of palladium on the copper surface.
 2. The method according to claim 1, wherein the reducing agent comprises a boron reducing agent.
 3. The method according to claim 2, wherein the boron reducing agent is selected from the group consisting of alkyl boranes, amine boranes, borane complexes, boron hydride compounds and combinations of one or more of the foregoing.
 4. The method according to claim 3, wherein the amine borane is selected from the group consisting of trialkylamine boranes, straight chain methoxy-substituted dimethylamine boranes, N-alkyl substituted morpholine boranes and combinations of one or more of the foregoing.
 5. The method according to claim 4, wherein the amine borane comprises dimethylamine borane (DMAB).
 6. The method according to claim 1, wherein the pre-dip composition is maintained at a temperature of between about 20° C. and about 30°.
 7. The method according to claim 6, wherein the pre-dip composition is maintained at about room temperature.
 8. The method according to claim 1, wherein the copper surface is contacted with the pre-dip composition for between about 30 seconds and about 2 minutes.
 9. The method according to claim 1, wherein the concentration of the reducing agent in the pre-dip composition is between about 2 and about 20 g/L.
 10. The method according to claim 9, wherein the concentration of the reducing agent in the pre-dip composition is between about 5 and 15 g/L.
 11. The method according to claim 10, wherein the concentration of the reducing agent in the pre-dip composition is between about 8 and about 10 g/L.
 12. The method according to claim 1, wherein no additional process steps are performed between steps a) and b). 